1. Field of the Invention
This invention relates generally to integrated circuit packaging, and in particular to flip chip packaging. More specifically, the invention relates to compositions and techniques for strengthening the interfacial bond between a flip chips package substrate and the solder balls used for external electrical connection of the completed flip chip package.
2. Description of Related Art
In integrated circuit packaging, a semiconductor die (also referred to as an integrated circuit (IC) or “chip”) may be bonded directly to a packaging substrate, without the need for a separate leadframe or for separate I/O connectors (e.g., wire or tape). Such chips are formed with ball-shaped beads or bumps of solder affixed to their I/O bonding pads. During packaging, the chip is “flipped” onto its active circuit surface so that the solder bumps form electrical connections directly between the chip and conductive traces on the packaging substrate. Semiconductor chips of this type are commonly called “flip chips.”
Integrated circuits are normally mounted on a printed circuit board (PCB) for use in an electronic device. Electrical connections of a flip chip package are generally made via a solder ball grid array (BGA) formed on the opposite side (referred to herein as the underside) of the packaging substrate from the chip. An unmounted flip chip package with a BGA arranged on its underside is aligned with a PCB and the package and board are then electrically connected in a solder reflow operation in which the solder is heated above its melting temperature and bonds with electrical contacts on the board upon cooling.
As semiconductor technology advances, the gate density of integrated circuits has increased. The substrate of choice for high density flip chip packages is produced by electroless nickel immersion gold (ENIG) technology. ENIG deposition processes provide flat, uniform metal surface finishes with excellent solderability, hardness, wear and corrosion resistance. As a result, ENIG processing has become widespread in BGA component packaging.
Unfortunately, it has been discovered that some solder joints in the BGA become fractured after surface mounting of the BGA package on a PCB where ENIG processing is used (See, for example, Goyal, et al., “Failure Mechanism of Brittle Solder joint Fracture in the Presence of Electroless Nickel Immersion Gold (ENIG) Interface,” Proceedings of the 52nd Electronic Component and Technology Conference, 732-739 (May, 2002)). The fracture occurs at the interface of a solder joint and the ENIG deposited surface. The cause of the fracture and/or susceptibility to fracture has been identified as residual phosphorus from the ENIG process that results in formation of a nickel-phosphorus layer at the nickel solder interface. Such brittle, interfacial solder joint fractures represent a significant reliability issue for high density flip chip package components. Nevertheless, ENIG processed flip chip package substrates have become standard in the industry and thus their continued use would be desirable.
Accordingly, what is needed are compositions and/or methods to reduce the occurrence of brittle, interfacial solder joint fractures at ENIG processed flip chip package substrates and PCBs.